Low-Cost, High-Energy Na-Ion Hybrid Supercapacitors

Iron-based phosphate [Na3Fe2(PO4)P2O7] cathodes have been regarded as potential cathodes for sodium-ion batteries (SIBs), but the poor rate capability restricts their fast charging performance. In this work, we have synthesized mesoporous Na3Fe2(PO4)P2O7 (M-NFPP@C) materials and employed them in rechargeable sodium batteries. The mesopores are produced by the decomposition carbonization of polyvinyl alcohol, which enlarges reactive sites and accelerates Na-ion migration. As a result, the reversible specific capacity maintains at 81 mA h g–1 (72.5% of 0.1 C reversible specific capacity) under an ultra-high C-rate of 40 C (equal to 1.5 min) and exceeds 1000 cycles life. A Na-ion hybrid supercapacitor using M-NFPP@C as a cathode and active carbon as an anode can achieve a maximum energy and power density of 48 W h kg–1 and 1350 W kg–1, respectively, with a capacity retention rate of 92% after 500 cycles at 20 C.